Edexcel A Level Chemistry:复习笔记5.5.5 Continuous Monitoring Method

Continuous Monitoring Method

 

  • Continuous monitoring involves collecting experimental data throughout the course of a reaction to plot a concentration-time graph
  • Two of the most common ways to collect this data are by:
    • Measuring the volume / amount of gas evolved over time
    • Measuring the mass of reactants lost over time
  • Another alternative method involves the use of colorimetry:

The iodination of propanone

  • The iodination of propanone provides a suitable experiment in which the rate of reaction can be measured throughout the reaction by using a colorimeter
  • The reaction is carried out using a catalyst of dilute sulfuric acid
  • The iodine decolourises during the reaction as it turns into iodopropanone and hydrogen iodide:

CH3COCH3   +   I2  → CH3COCH2I   + HI

  • The colorimeter measures colour absorbance which is proportional to the concentration of the coloured species
  • Before the investigation begins it is necessary to  measure the absorbance of a set of standard solutions of iodine and obtain a calibration curve
  • For example, here is a calibration curve for a transition metal ion that allows you to convert colorimeter readings into concentrations:

 

Visible-Spectroscopy-Calibration-Curve

A calibration curve showing the relationship between colour absorbance and concentration

  • The colorimeter uses very small volumes of solutions, so four burettes can be filled with solutions of 0.02 mol dm-3 iodine, 1.0 mol dm-3 propanone and 1.0  mol dm-3 sulfuric acid and distilled water
  • By varying the volumes of solutions while maintaining a constant total volume with the use of distilled water, you can obtain a number of different concentrations
  • The solutions are measured into a small beaker, leaving the iodine in a separate beaker - this starts the reaction, so it can be added when you start a timer or stop watch
  • The iodine is added to the other liquids, the contents mixed and then quickly transferred into the cuvette (small receptacle) and the colorimeter / data logger started

6.1.3-Colorimetry-Set-Up-1

The set up for using a colorimeter and data logger to continuously measure the rate of reaction

 

  • A typical set of volume compositions could be as follows:

Volume Compositions Table

8.1.1-Continuous-Monitoring-Volumes-Table Practical tip

  • Choose a filter that gives the strongest absorbance for the solution you are using - this will be the complementary colour to the colour of the solution under investigation

Specimen results table for the iodination of propanone

8.1.1-Continuous-Monitoring-Results-Table

 

8.1.1-Continuous-Monitoring-Results-Graphv

 

Graph showing the change in concentration of iodine during the course of the reaction

Measuring rate

  • To find the rate of reaction at any point, a tangent is drawn and the gradient is determined
  • The gradient gives the rate of reaction
  • For example, in the graph above, the rate of reaction at 300 seconds can be found
    • A vertical line is drawn from the 300 s mark until it meets the curve, then a tangent is drawn
    • Gradient 1.19 x 10-5 mol dm-3 s-1
    • The gradient is the rate of reaction at that point

Exam Tip

Whichever rates experiments you carry out, make sure you can

  • describe the steps in the procedure
  • name all the apparatus used
  • draw data tables which include headings and units
  • draw graphs showing labels, units and best fit lines
  • determine an initial gradient or at any point in the curve

转载自savemyexams

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